This is really the business end of the design. Thermal gradients and stresses are at a maximum ( within a specified design factor of safety ). The main purpose of the receiver is to collect the thermal energy and send it to the prime mover. The maximum efficiency (according to my thermodynamics class) comes at the the highest temperature differential. I.E. the temperature delivered to the engine vs. the ambient or rejection temperature. Proper insulation is obviously important to keep the heat captured, within the system itself and not allow it to escape back into the cold surrounding environment. Obviously if there is a way for the heat to enter the receiver boiler assembly, then there is also a similar way for it to escape back out. I have been wrestling with this for a while now and it occurs to me that the first consideration is primarily the aperture input / output size that allows the solar flux to enter into the receiver. If the image thrown from the mirror is the smallest possible, then once the sunlight has enters into the chamber and starts bouncing around giving off heat to  the inside, it will be harder for it to find the hole to escape.

    A second idea might be to install a kind of one way mirror used in laser applications that will allow the light to come into the chamber and then reflect it back when it tries to escape. This will obviously generate a great deal of heat because of the flux density so cooling water will need to be preheated and passed over / under or around it to absorb any excess heat while at the same time keeping the thermal gradients to within acceptable levels to prevent cracking.

    The main problem with any glass in the system is the low melting point compared with the structural alloys. Glass melts at around 1200 °F and is significantly weakened in strength for several hundred degrees below the melting point. So if any of this material is used for any purpose it must not support any significant loading, and the temperature kept probably to around 800 °F.

    According to the 2001 ASME Pressure Vessel and Piping Code, we have some ferrous nickel alloys that have the ability to operate at 1650 °F and 1.8ksi of stress. We want the highest temperature and pressure achievable based on the lowest amount of generated entropy for whatever fluid that we use to carry heat. Neutral salts (or liquid metals) can absorb high temperatures without generating a great deal of vapor pressure, and would seem to be a fair choice to transfer heat energy from the  receiver to the engine. This will require the addition of a pre heater to bring the piping up to operating temperature prior to flowing the carrier in the morning at sunrise.

    Going back to my early naive choice of steam turbine, obviously heating water to 1650 °F will generate a great deal of pressure and the pipe material and walls will need to be significantly stronger than if it were to carry a neutral salt. On the other hand finding a salt that operates at this high of a temperature may be difficult because they tend to break down at elevated temperatures, and are more prone to dissolve any surrounding metals.

    Another problem with high temperatures is that if there is any hole whatsoever in the insulation, any crack or air flow next to the high temp equipment, the heat is going to find a way to get out. If heat escapes then your power escapes with it,  and at higher temperatures you get a significantly higher heat flow rates which means that the same quantity of insulation will be less effective.

    Any way the main point here to develop as many ideas and configurations as possible, and then run the various configurations through the numbers, try to balance the energy flow and equations to maximize output, and then see which designs come out on top of the price / performance ratio.