A very cheap flash box can be made by cadging a polystyrene box from a local shopkeeper. Tropical fish are imported in similar boxes . I got mine from a friendly greengrocer that had his chilled brocoli arrive in them.
Cutting an A1 sized sheet of paper can get a good seamless background and for taking pictures of light coloured objects or clear glass a tinted paper gives good contrast to the background.
Holes are cut slightly smaller than A4 copy paper and the sides are then covered, the flashes are positioned at around 9-10 inches away with diffusers on them. This gives a good spread of light on the A4 paper and to the inside of the box. I am lucky in that my old Minolta 360 px flashgun will link to the Sony Alpha camera via an adaptor and my Vivitar 383 flash is fired via a slave.
It should be possible though to fire both flashguns via slaves by covering most of the on-camera flash just leaving enough light to trigger the slaves, If the guns are too powerfull and cannot be stepped down then all that is needed is to move them further back from the box.