I use 60 UV LEDs, no box, just hold them about 30cm above the board and move them around a bit to get even exposure. It works fine for boards up to 10X16 cm, I plane to make another set if I want to do larger boards.
You need to check the specs of the LEDs and make sure the wavelength matches the resist. Got them on fleabay, quite cheap.
It could be a simple and good option because I can also use positiv20, unless the UV leds were expensive here
https://www.conrad.de/ce/de/product...lett-Min-407Max-412-nm-Ausfuehrung-Wasserklar