Not sure this has been raised before but I have had plenty of help from this Forum so trying to give a little back : (Think this only applies to Series 1/2 Owners)
Have recently finished a series 2 build and worked out that anyone can lift off my panels and walk away with them and estimate about £300 a pair to replace.

(Adding a large lock and chain doesnt help !)
Not wanting to drill additional holes for locks that spoil the original look came up with this simple idea.
Drilled a small 4mm hole through the housing that the panel handle locks into and purchased some stainless Clevis Pins from Ebay £2.59. Once the panel has been closed these can be inserted through the housing and locked in place with the R-clip supplied.
https://photos.app.goo.gl/HY8XZYEJCnHao3py6A little messing about with some washers and spring washers packed out the gap and stopped any movement or rattles. Added some grease to stop any rusting.
I know it is not a permanent security fixture but would deter a thief as would probably take a few minutes to work out how to remove the pins. (The pins can be removed at the roadside without any tools so access is not heavily hampered.)
If you wanted to go a step further you could use a split pin and bend it over, unlikely for someone without tools to be able to remove easily.