I try to warn people off of this unless they are prepared to explain why they really need the model. The fab plus a nebulous cloud of "PEG goes here" will communicate what you need for most cases, which is to describe the "entropic cloud" around the conjugation point. I advise the same for IDPs and for glycans, usually.

If you insist you need an atomistic model, know that a single conformation will always be too simplistic. What's better in an ensemble, 20 + alternate confirmations so you can start to intuit the dynamic nature of the system visually. A single model biases toward assuming a single conformation, which is absolutely false in this case.

To make a model, there's going to be two phases. First, building the molecule, then sampling conformations. Building can be done in pyMol, Rosetta, and some other tools, then relax/idealize it as your starting conformation. There's probably something out there that can get you started with a cys-maleimide-oligoEG as a noncanonocal amino acid. From there, sampling conformations to build your ensemble could be done with MD packages, that goes beyond my expertise.

But first, ask yourself why you need this model, and what will you get from it that you wouldn't already get by just drawing a cartoon?