There's a thread here on the use of fondant - which is one good option:
https://beemaster.com/forum/index.php?topic=50659.0If you're staying wih syrup, then it's important to keep the sun from heating the jars - as it's the liquid expansion (cooling followed by warming) which tends to progressively push syrup out through the holes - so some form of 'feeder shell' around the jar is the cure for that one. I also use insulation around the jars to keep their temperature as constant as possible.
With nucs, use the smallest jars you can find. I currently run 5-frame nucs with deep frames rather than mini-nucs and use re-cycled 1 lb jam-jars as inverted jar feeders, with four small holes (three would be ok) surrounding the central 'safety (tamper-proof) indicator'. I used to smear a tiny amount of petroleum jelly onto the lid seal, but don't bother these days as it can be a tad messy - although the lids do now sometimes stick fast. Whenever this happens, then I simply do a 'jar swap', and at the end of the day run boiling water over the stuck lid which always releases it.
When working with inverted jar feeders - you can test the seals easily enough by filling the jars half-full with water and inverting them over the kitchen sink. Shake downwards once or twice - then hold the jar there inverted and wait to see if the dripping stops. If it doesn't - then check the lid seal and jar lip carefully for damage or imperfections. Any 'iffy' ones - bin 'em.
Like yourself, I find ants to be a pain in the butt - some of them even manage to climb into the jar via the holes and die there. I don't yet have an answer to this ... but I'm working on it. It'll probably be something along the lines of a grease band fitted to the stand legs ...
LJ