... not sure how much actual D power you need, and how big a payload section... you could put a 24mm MMT in the booster of Estes Echostar. it's heavy and has lots of drag from those fins so needs the boost. it has a BT20 sized clear payload section.
I can't give you a Payloader Kit, but a Can offer a source for 48" Clear polyethylene mailing tubes with a .93" ID, .97" OD with a .02" wall, The stuff fits onto standard BT-50 balsa block and is a tight fit on Jt-50 couplers. I use these tubes to make new and replacement payload bays and scratch Phantom flying bodies
McMaster Carr #2044T46 page 1388 at 1.11/4ft piece.
Hope this helps.