i got a ebay package of 4 passive radiators (4"), sadly they didn't include a mechanism to add weight to them easily.
search ebay for the radiators with this term:
4 inch Horn Radiator Woofer Speaker Passive Radiator Auxiliary Bass 105x105mm
they cost about 8$ for a pair at max
since i wanted to test with and without extra weight, i decided to glue a 80g 0.75mh coil on one radiator on the inside to measure the differences.
without extra weight: a huge bump (about 10db) at 200hz and nearly no real bass response.
then i swapped to the radiator with inductor, and found out that the radiators glued in inductor was touching the TB speaker's magnet, which meant the enclosure isn't deep enough.
a temp solution i applied was mounting the passive radiator backwards.
then i measured, and found out that the speakers now have a very nice bass response, especially for their size, they sound like bigger bookshelf speakers in a room of 8*12 meter, and just one speaker (mono) can easily fill the room with a very rich experience sound (almost as if you were in the recording studios), they go pretty low as well: down to 55HZ with ease (lower than i expected)
the response at 1 meter distance, in the 8*12meter living room, please don't go lower than 40hz, as the distortion starts to increase by alot (average 10% at the rms wattage, where 45-25000hz is 2% distortion at the rms wattage)
and for the radiator depth issue, i ordered a new backplate, one which is 12mm deeper than the original, meaning i can mount it as it's designed when it arrives :) (should be 7th of june)
costs / speaker right now is: 160/piece (when ordering the enclosured at 3dhubs), which can be reduced alot if you're going for MDF wood instead of 3d printed (i printed the models in colorfabb ngen)
costs would then be more like: 110 a piece.
anyhow, i hope this'll be enough regarding this update :P
going to update project files right now.