Hi B,
Maybe a simple close up lens used for macrophotography would work. they are available from +1 to several dioptres. Since you will be using them with eyepieces you might want them in smaller diameters, which is typically 37mm at the low end. You could then perhaps just use the 37mm lens with its own thread and then 3D print an adapter to suit you eyepiece if the eyerelief distances and magnification suit. It would be an experiment. Here are some links for eBay 37mm close up lenses....
Best
JA
https://www.ebay.com.au/itm/28497368...Bk9SR_bHjITLYQ
https://www.ebay.com.au/itm/38541896...Bk9SR_bHjITLYQ