There is some kind of rubber paint I have seen on DIY TV used in planters to make them waterproof. It is black anf they just painted it with a brush and once dried it was totally sealed but don't remember to leave your drain holes. They plugged them until they finished painting and them unplugged them before the paint sealed them so they could get them out. Drain holes toward the front so that the water didn't drain againt the house causing rot.
I am wondering if $300 is a typo? How many and how big are they?
You can make your own out of fiberglass mat and resin if your handy, or simply go to a HVAC contractor and have them make metal ones out of galvanized sheet metal. When I had my metal shop we made them all the time for folks in our area.
That seems high. This company has fancy "self-watering" ones for a 20-inch planter for $200:
They are 18x18x48 in fiberglass. Does the galvanized hold up ok? I also read that the some metals react to the plants? The rubber paint sounds good to if it doesn't leach out chemicals. How much are the metals one? Do you still make them?
Use thick plastic, line them and staple just under the edges. Poke holes or slits in the bottoms and use some gravel for drainage. Cheap and you can replace as needed.
We made them for around $60 each. They were three feet long six inches wide on bottom and about eight wide at top.
Most people simply sprayed undercoating to make them resistant to any rusting although the galvanized metal resists rust for many years.
No I no longer have a metal fab shop, although looking for some equipment to do some smaller work. Any local Heating contractor in your area should be able to make them to your size specifications.
You could also coat the inside with fiberglass resin. There are lots of web sites that sell this type of product and its just a matter of mixing and brushing it on. Let it dry and your done.
What about the rubber type liner, on rolls you buy by the foot/yard, for Koi Ponds etc.?
Posh, at Lowe's there is a rubber paint in the spray paint dept. that is in cans and you dip the handles of your tools in to give them a softer colored grip. You could paint this. Also, there is the black rubberized paint in spray cans. It seals pipe leaks, garden hoses, roofs, etc. This seems the easiest and most cost effective. I would give this a try. I think it's about $10 a can. Once dried, it won't affect your plants. Make sure to leave you some drainage holes to the front of your planter boxes.By front, I mean further away from your exterior wall.
The rubber paint seems to be the best option. I was just worried about the leaching from the rubber. Rubber rolls are good to but the time it takes to cut and seal makes it cheaper to buy the fiberglass.
I cut pieces of tin to fit the bottom of the long large porch railing flower box and riveted the tin together, then drilled holes in the bottom where I had added holes in the wood when making the planter. That was about 30 or more years ago and still looks great.