Lang, The most compact small Joey box I have seen is the one from the 6 cylinder Morris about a 4 tonner, a mate of mine has one in his Morris and it appears to be quite strong, can be fitted either way, to speed up or slow down the drive shaft. Mind you, the Morris does have enough power to spin it also !
I noticed a joey box listed on gumtree website the other day.
Listed as "Gear Box", but i just found it's quicker to find the advert if you search for "joey box".
Advert says "ACCO 1800 series joey box 3 speed $500". Loxton/Waikerie area SA.