Lettuce and most greens are easy since they tend to grow "bushy" and fill "out" over the container. Tomatoes and other fruiting plants tend to expand vertically in a bigger proportion than horizontally and considerable force is then exerted on the small container base. Since the roots are "free floating" and provide no real support, the risk of a plant toppling is real and can cause many problems not only to the plant, but to the system itself and other plants in the area.
An easy technique is to find the appropriate size tomato "hoop" as can be found in any box store or garden center. These work well "upside down" because they provide a solid base on the floor. Measure first to insure the bottom hoops will fit over the reservoir and onto a solid, flat surface. Be very careful of the exposed ends since they will now be on top and at eye level. Bending them inwards and curving downward should alleviate this hazard. Square hoops work as well, but again, insure the hoop will fit before purchasing.
But since this is about "frugality", a stick trellis is probably the best method for not only cost, but flexibility in size and strength. Gather three to four 6'-8' sticks or branches of about 1" - 2" diameter. Lay them out in the sun for a few days to dry and expose any bugs that have hitch-hiked to birds or to the sunlight. This is especially true if you will be using them indoors. Trim any remaining branches and bark to aid in tying the plants to the supports.
Once dried and ready, simply build a "teepee" around the base of the reservoir or container and fasten the meeting point with twine or wire. Some bracing may be needed around the base to prevent the legs from sliding, but once there is weight from the plant they tend to stay in place quite well.
As the plant grows, use discarded nylons torn into strips as a means to secure the plant to the legs. Another option is cheap yarn, but the key is to find a material that is flexible and will not cause abrasions to the plant such as would wire, zip-ties, or zero-stretch string.
Use your imagination in suport techniques and allow the plant all the room it needs to grow while preventing the dreaded "tip-over"!!
