U set the aperture, then only dismount the lens. That way, u get the aperture u want

Can, u have to press the apreture button while dismounting it...
go try it..
hehehe!

I using no cost method (no extension tube,ring,dioptic or watever), follow the steps:

1. set the aperture to any aperture u want, then press n hold the depth of field button (for canon, its found near your dismount lens button) then use another finger to press the dismount button, u can see ur aperture blade didnt return to default.
2. Im using 50mm f/1.8 therefore, the front lens can be insert in, but still need to hold it but atleast the lens wont easily move XD.
3. Im using external flash mount on top of my camera with manual control of light n diffuser panel.

eh? how did u open up the aperture if you put the lens on reverse? wouldnt it be at smallest aperture? (did u use any pin/mechanism to keep the aperture opened?)