For a Roborovski 8cm diameter or larger. Has he only just started doing this or has he always done this? In theory a bigger wheel should help prevent them going right over the top with centrifuge. But sometimes they just leap out.
A good option can be the 9" silent runner wheel. The holes in the front tend to make them stop and check before jumping out

You can get bar attachments for it so it fixes to the cage bars at whatever height you want.
BOGO Free - Yellow Silent Runner 9 in. Wheel No code needed. Simply add 2 yellow wheels to your cart and discount will be automatically applied. One free wheel per order, while supplies last. Looking for a safe, quiet, and durable exercise wheel for your furry friends? Look no further than...
exoticnutrition.com
Bar attachment plates (need buying separately)
Silent Runner Cage Attachment includes two brackets used to attach your Silent Runner Wheel directly to your wire cage. One size fits all Silent Runner wheels. Does not fit other wheels designs (i.e. Wodent Wheel). The new & improved Cage Attachment Brackets are a larger size than the previously...
exoticnutrition.com
It works really well attached to the bars and it's a completely silent wheel.
@hamster lover 123 's Robo, Dora has this wheel