SVBony
I have a more expensive Prima Luce one for the main scope and I decided to buy a SVBony one off ebay for the guide scope. The latter is USB powered, has three power levels and I think I paid $30 all up. The Prima Luce was $109 which also required a power regulator which was $69.
Crazy.
A 400mm strap (your 115mm scope should have a circumference of 360mm) costs $38 on ebay
https://www.ebay.com.au/itm/SVBONY-S...frcectupt=true