Used 4x5 film holders are quite common on eBay. Not sure how eBay might work in Poland but used holders of reasonable quality should be $10US or less.
Browse at
www.f295.org and you will see how some people are using cigar boxes as the camera body. Others build a 5 sided box and the film holder becomes the 6th side.
Keeping the film holder attached to the camera can be as simple as rubber bands (elastic bands).
To design your own, start with the film holder and measure pieces to make a 5 sided box. Look closely at how the film holder is constructed so you can take advantage of its shape and light-trap features.