Building the Backstop

Update: it is now recommended to follow these instructions except layered with two of these tarps (can even share the same bungees if the grommets line up).  This seems to make them last longer.

We will begin by making the backstop which serves as a projector screen and arrow stop.  This part is quite simple, we will first just make a rectangle frame with our EMT and 4 of the Makerpipe T connectors and then string our tarp across the frame.  Here is what it will look like when completed:

First, unfold and measure the tarp pulled taut.  It is important that the frame we build is larger than the tarp so it can be suspended in the middle by the bungees.  A roughly 3” gap all the way around works nicely.

Tarps often come in a measurement of “cut size” or “finished size”, with the “cut size” being the measurement before the hemming is completed and the grommets are installed.  Allowances for hems and such often amount to right around 6”.  This means that if you purchase a 6x8 “cut size” tarp, it will actually be around 5.5x7.5.  Thus, if we make the frame to be 6x8 (interior dimensions), the tarp will be the perfect size to leave around a 3” gap on every side. 

If your tarp is a different size, you will want to just add 6” to the actual length and width of the tarp to get the interior dimensions that you should target for the EMT frame.

Start by cutting the height.  Since we are shooting for an interior dimension of 6x8, we’ll need to add the thickness of the EMT to both the top and the bottom.  Rounding for simplicity, ¾” EMT has an outside diameter (OD) of around 1” (0.92” actually but close enough).  Thus you’ll want to cut 2 pieces of EMT to 6’2”, one for each side, and use a grinder to remove any sharp edges or burrs.

Cutting the width is a little less straightforward since the T connector adds (again rounding for simplicity) about 1.75” on each side.  So if our target length is 8’2” (OD), we then should subtract 3.5” to get a cut length of 7’10.5”.  Cut and deburr two of these.

Lay these cut pieces out in a rectangle on a flat surface so the joints are square as you put it together.

Use the Makerpipe T connectors to form the rectangle.  Note the orientation of the connector in the diagram below.  The side to side conduit should slide into the part of the connector with the bolt, whereas the vertical conduit should be "grabbed" by the "puzzle pieces" of the connector.

Once the rectangle frame is formed and all connector bolts are tightened, place the tarp in the middle and string it up suspended in the frame using the ball bungees.  Use one bungee per grommet.  If the tension is too loose for the tarp to remain suspended  in the center of the frame when set vertically, or if the ball bungees do not have enough tension to maintain their hold, you should wrap them completely around the frame and then fasten.  I would recommend only doing this as necessary so that you can do this later to tighten as the bungees stretch over time.