The marching pack is described by Plutarch and Josephus, and is shown on Trajan's Column, but few remains have been found. Details of these items are therefore uncertain and you may feel free to deviate from the following specifics as you need or desire.
The pack items are carried on a T-shaped pole about 5 feet tall with a crossbar approximately 18" long. Construction details are unclear, but the crossbar is best secured 3-4" from the top of the pole with a bolt or big nail (cleverly filed or hammered to keep it from looking too modern). Wrap the joint with a leather thong to steady it.
From the crossbar hangs a bundle which is presumably the cloak. It can simply be rolled up and tied, wrapped in a piece of leather or cloth, or held in a bag which is secured to the crossbar. Some of the bundles shown on the Column are tied at one end, some at both.
The rectangular satchel measures approximately 12" x 18" and is made from 1-3 ounce leather (goatskin, etc.). On the column it is shown as flat, with no side gores. The best working reconstruction is reinforced with leather strips and has a pointed flap like an envelope. The reinforcements form a * shape on the back, but on the front they hold a ring which serves as the flap closure. If a plain ring is used the flap can simply be tied to it with a thong; however, bronze rings with raised studs have been found which may serve this purpose, the stud serving as a button. The side reinforcements (or the crossed strips, in Simkins' example) hold a ring at each upper corner to which the strap is anchored. At the middle of the top edge is a carrying handle which can be reinforced with a cord inside it. The strap and handle allow the satchel to be hung from the T-pole in a number of ways, or slung from the shoulder when "marching light". (In fact, one could even substitute a dolabra or entrenching tool for the T-pole, hanging the kit items from the tool's head.) The fragmentary satchel found at Bar Holl apparently had no flap, and may have been a mule's feed bag, but it is a handy reference for some construction techniques.
The most mysterious object shown on Trajan's Column looks like a net bag, which is not mentioned in literature. While it could simply be a net bag (for food items?) it could also be a reinforcement either for a linen sack (for grain or flour?) or for a leather water flask. The latter interpretation is popular since it is not known how else the Roman soldier might have carried his water. The net may be made of linen, jute, wool, or similar cord, but its exact form and function depends on your personal interpretation of the evidence.
The body can be made of one piece of 1-3 ounce leather, or from several pieces. Reinforcing strips are also 1-3 oz., 1/2" - 1" wide. Stitching can be a running stitch or 2-needle method. After the body of the satchel is assembled and crossed reinforces are sewn on, sew on a reinforcing strip around the sides and bottom edge. At each top corner it is doubled back through a plain metal ring (1 1/4" to 2" diameter) and stitched down. The handle is sewn to the middle third of the top edge; reinforcing patches inside the satchel are recommended. The edges of the flap can have a turned or bound hem.
Optionally, the crossed strips can hold the corner rings and the "perimeter" strips omitted, as on Simkin's reconstruction.