I built a modified chair that sort of worked, for a 10yo. Then I trashed it for parts when the speed controller died. Time to iterate.
The frontpage of wheelchairdriver.com was invaluable for me select a 2nd version of a used chair and upgrade it using what was in front of me.
Learnings: I understand about FWD, MWD and RWD now. Experienced it and wouldn't curse anyone with FWD or MWD. Transferring is a priority in our design. Both armrests and footrest. Modified a MWD footplate and built a mount to a RWD, taking into account @burgerman advice for caster placement and foot placement. Used a bariatric seat base from Jazzy and cut it smaller to give comfortable, easy to move armrests with a great hand position, combined with a sport seat that makes it very difficult to slide off not desired. The person for this has good core strength and needs to be encouraged to use it, even though his legs don't.
End result: You're decision to share this info, made a kid's year. I got v2 done for his birthday, today and it's an amazing fit. Thank you for helping me remember to think of design from the driver's perspective. Not my "you'd better be grateful".
The picture is just as he got it. I was able to adjust the footrest up, joystick up and back and the backrest forward.
Can I do more: yep. But thank you. Really.