I need to join two lengths of 4mm armoured cable - above ground - in
fact I'll need three junction boxes in total.

I know I can get this sort of thing - https://tinyurl.com/yc37e53k - but
this is a favour for someone else and I want to try and keep costs down.
Any recommendations for cheaper IP66/67 boxes that will be robust and
durable enough, and which will be a good deal less that £60 in total?

I should add that this cable is to run along a garden border about a
foot off the ground.