I haven't had any problems filling twist-up tubes for lotion bar or lip balm products. I've filled typical small lip balm tubes (0.15 oz, 4 g) to medium sized cylinders (0.5 oz, 15 g) to large deodorant ovals (2.65 oz, 75 g). I have heard of people who do have troubles with leaks and the plastic tubes distorting, but I've gathered the most common reason why that happens is they are filling the tubes when their product is smokin' hot.
I use a hot water bath (bain marie) and keep the water bath just warm enough to melt my ingredients. That amounts to a water bath temp of 170-180 deg F (75-85 C) for a beeswax based recipe. If you use veg wax, you can probably go cooler yet. When I pour the balm/lotion into a tube, the cool plastic causes the product to congeal pretty quickly on the walls of the tube.
I buy the small and medium sized tubes from Majestic Mountain Sage and I use their filling trays to fill these tubes. Works a treat. MMS is based in Utah, USA, so that won't be helpful to you, but they do offer these tips on their website:
"...We have a couple of hints for users.
"1) Pour the lip balm mixture as cool as possible. The tubes can become mis-shapen if the lip balm mix is super hot. Use our quick test to determine if your lip mixture is ready: stir the mixture with a pipette, hold the pipette horizontal after stirring, if the pipette bends downward then the mixture is WAY too hot. If the pipette holds it's rigid form and the mixture is still clear then the mixture is at perfect pouring temperature.
"2) Load the tray with tubes just before filling. Long term storage of tubes held in the tray can cause temporary distortion of the tubes. To return tubes to their original shape, remove them from the tray and allow them to sit without their caps for a few days.
"3) Allow the filled tubes to fully cool before removing from the tray. This may take a hour depending on the temperatures of the lip balm mixture and the room where you are working. Then, when the tubes are cool, give the tube a twist just before pulling straight out. The lip balm mixture at the top of the tube (the glorious crown!) can be damaged if the lip balm is too soft when removed from the tray. Patience is certainly an asset in this case...."
Source:
https://www.thesage.com/catalog/LipBalmContainers.html
I hope this helps!