Very nice! I did something similar with my XP4. My daughter is 3 years old and I too have a PRP booster for her, but I feel she is still to small to fit in it properly and to get the seat belts to fit her right.
So I bought another car seat like we used in our passenger cars. (Graco Nautilus) To mount it in the XP I used the factory seat base, made an adapter plate that bolts to the factory seat base and that the car seat mounts too. Since car seats come with "Latch" tethers to strap it in your car. I added two eyelets and also used the Latch tethers as a secondary hold down. I too can take her seat out and put a full size seat back in within a few minutes.
Here is the Graco Nautilus seat. I like it for the fact the head rest wraps around protecting her head in side to side movement and is adjustable. Also the shoulder belt height is adjustable so you can tailor fit the belts to fit their size perfect. Also the seat gives them a to handles to hold on to.
